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Leveling OSB underlayment

I am putting down a laminate floor in my bedroom and have discovered a 2'x4' piece of the OSB subfloor, up against a non- load bearing outside wall, that has a dip in it. In the middle of the section, up against the wall, its 3/8" below the surrounding floor and it fans out to level at the inner edges of the piece. There is no sponginess to the floor or creaking when I walk on it and I haven't found any damage to the joists or structure below the subfloor. I assume the rim joist is just cupped beneath that section.

I can't lay the laminate over this area because it floats in midair. I wanted to use a leveling compound over the OSB to bring it up to level but the manufactures that I spoke with told me not to use the stuff unless I'm sure the OSB is exterior grade. The leveling compound will soak into non-exterior grade OSB and cause it to delaminate. None of the OSB sheets has the factory marking showing and so far I haven't been able to find a marking on the underside by pulling down insulation in the crawl space. I can't replace the OSB section because the outside wall is sitting on it.

Does anyone have a better method of leveling the OSB so I can finish laying my floor?

Re: Leveling OSB underlayment

I found a solution to my problem so I thought I would share it.

After some searching, I exposed the manufacturer's stamp on the underside of my OSB but it didn't say anything about being "exterior grade". I called the tech support number for the manufacturer, Huber, to get their recommendations. The technician said the product wasn't exterior grade but should hold up to most leveling compounds. He did recommend starting out with a skim coat to seal the OSB with minimum water exposure.

I followed Huber's recommendation, putting down a skim coat followed by two thicker coats to achieve the required maximum of 3/8" and edge feathering. The material, Henry Products 749, worked well except that I had to thin the mixture for the second and third coat. The recommended proportion resulted in a mixture that was just to stiff to feather. I let the coats stand for 24 hours between applications and before laying the floor.

The final outcome was acceptable, the foam backing on the laminate flooring makes up for any small irregularities. When walking on the leveled section, one cannot tell the difference from the rest of the floor.

A. Spruce
Re: Leveling OSB underlayment

Had I seen this sooner, I'd have offered the same advice. OSB, while being a poor product for any purpose, isn't going to delaminate with the minimal amount of moisture that is in floor leveling compound. Leveling compound is used all the time over particle board, which is even worse than OSB when it comes to getting wet.

At any rate, glad you got it figured out.

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